Ionizing radiation is a well established treatment modality for malignant disease and is of proven benefit for both curative and palliative purposes. The complications of radiotherapy are well known and include mucositis, leukopenia, desquamation, spinal cord necrosis, and obliterative endarteritis (Grosch, D. S., Hopwood, L. E., Biological Effects of Radiation, Academic Press, New York, San Francisco, 19791 Peters, L. J., Thawley, S. E., Panje, W. R., ed. Comprehensive Management to Head and Neck Tumours, W. B. Saunders Philadelphia, London, Toronto, 1987 pp. 13-152; Petkau, A., et al. Life Sci. 1978:22, 867-882). These complications frequently limit the ability to deliver a full therapeutic dose of radiation or cause significant morbidity following treatment. Thus, there has been an extensive search for agents which will either protect normal tissue from radio-injury without sparing the tumour, or will selectively sensitize malignant tissue to allow lower doses of radiation to achieve the same therapeutic effect with less damage to healthy tissues (Hall, E. J. Radiobiology for the Radiobiologist, 2nd edition, Harper and Row, Hagerstown, N.Y. 1978, Coleman, C. N., Seminars in Oncology, 16:3 169-75, 1989).
Ionizing radiation acts by displacing an electron from the outer shell of a molecule or atom, resulting in a species that is electron deficient (Grosch, D. S. Hopwood, L. E., Biological Effects of Radiation, Academic Press, New York, San Francisco, 1979; Clark I. A., Pathology, 1986:18,181-186; Thomas, J. K. Silini, G. ed. Radiation Research 1966:179-194; Southorn, P. A., Mayo Clin Proc. 63:381-389 (1988)). These electron deficient species are known as free radicals and are extremely unstable, reacting rapidly with adjacent molecules and atoms, causing alteration in their chemical structure (Grosch, D. S. Hopwood, L. E., Biological Effects of Radiation, Academic Press, New York, San Francisco, 1979; Clark I. A., Pathology, 1986: 18, 181-186; Thomas, J. K. Silini, G. ed. Radiation Research 1966: 179-194; Southorn, P. A., Mayo Clin Proc. 63: 381-389 (1988)). This chemical injury mediated upon DNA, particularly injury resulting in double strand DNA breaks, is responsible for tumour cell death in clinical radiotherapy (Grosch, D. S., Hopwood, L. E., Academic Press, New York, San Francisco, 1979; Peters, L. J., Thawley, S. E., Panje, W. R., ed. Comprehensive Management of Head and Neck Tumours, W. B. Saunders Philadelphia, London, Toronto, 1987 pp. 132-152, Clark, I. A., Pathology, 1986:18, 181-186; Hall, E. J. Radiobiology for the Radiobiologist, 2nd edition, Harper and Row Hagerstown, New York, 1978; Southorn, P. A., Mayo Clin. Proc. 63: 381-389 (1988)).
Radiolysis of the water molecule produces a free electron (e.sup.-), a hydrogen ion (H.sup.+), and the hydroxyl radical (OH)--a potent oxidizing agent (Grosch, D. S., Hopwood, L. E., Biological Effects of Radiation, Academic Press, New York, San Francisco, 1979; Thomas, J. K., Silini, G. ed. Radiation Research 1966: 179-194; Southorn, P. A., Mayo clin, Proc. 63: 381-389 (1988); Grezlinska, E. et al, Int. J. Radial. Biol. 1982:41, 473-481; McLennan G., Autor, A. P., Autor, A. P. ed. Pathology of Oxygen, Academic Press, New York, 1982, pp. 85-97; Petkau, A., Br. J. Cancer 55:Suppl VIII, 7-95 (1987)). The free electron can then be bound by other molecules to form additional unstable radicals (Grosch, D. S., Hopwood, L. E., Biological Effects Of Radiation, Academic Press, New York, San Francisco, 1979; Clark I. A., Pathology, 1986: 18,181-186; Southorn, P. A., Mayo clin, Proc. 63: 381-389 (1988); Grezlinska, E. et al, Int. J. Radiat. Biol. 1982:41, 473-481; Petkau, A., Br. J. Cancer 55:Suppl VIII, 87-95 (1987)). The most important of these is the reaction with molecular oxygen to form the superoxide radical (O.sub.2.sup.-) (Grosch, D. S., Hopwood, L. E., Biological Effects of Radiation, Academic Press, New York, San Francisco, 1979; Hall, E. J. Radiobiology for the Radiobiologist, 2nd edition, Harper and Row, Hagerstown, New York 1978, Coleman, C. N., Seminars in Oncology, 16:3 169-75, 1989); Clark I. A., Pathology, 1986:18,181-186; Grezlinska, E. et al, Int. J. Radiat. Biol. 1982:41, 473-481; McLennan G., Autor, A. P., Autor, A. P. ed. Pathology of Oxygen, Academic Press, New York, 1982, pp. 85-97; Petkau, A., Br. J. Cancer 55:Suppl VIII, 87-95 (1987)). These two radicals are responsible for most of the DNA injury mediated by radiotherapy. They do not discriminate, however, and damage to non-DNA structures, particularly the peroxidation of lipid membranes, cause many of the acute side effects of radiation without contributing to tumour cell death (Grosch, D. S., Hopwood, L. E., Biological Effects of Radiation, Academic Press, New York, San Francisco, 1979; Southorn, P. A., Mayo clin, Proc. 63:381-389 (1988); Grezlinska, E. et al, Int. J. Radiat. Biol. 1982:41, 473-481; Petkau, A., Br. J. Cancer 55:Suppl VIII, 87-95 (1987); Peters, L. J., Thawley, S. E., Panic, W. R. ed. Comprehensive Management of Head and Neck Tumours, W. B. Saunders Philadelphia, London, Toronto, 1987, pp. 132-152; Misra, H. P. and Fridovich, I., Arch Bioch Biophs 1986:176, 577-581).
The presence of dissolved oxygen significantly increases free radical generation and thus the effectiveness of radiation injury--the so-called oxygen effect (Grosch, D. S., Hopwood, L. E., Biological Effects of Radiation, Academic Press, New York, San Francisco, 1979; Hall, E. J. Radiobiology or the Radiobiologist, 2nd edition, Harper and Row, Hagerstown, New York 1978, Coleman, C. N., Seminars in Oncology, 16:3 169-75, 1989). The resistance of hypoxic tumours to radiation injury is well recognized in clinical practice and is a reflection of this effect.
A variety of radiosensitizers have been developed and studied. The majority of these are nitroimidazole compounds such as metronidazole, misonidazole, and etanidazole (Hall, E. J. Radiobiology for the Radiobiologist, 2nd edition, Harper and Row, Hagerstown, New York 1978, Coleman, C. N., Seminars in Oncology, 16:3 169-75, 1989; Schor, N. F., Biochem pharmacol 1988:37(9), 1751-1762). Peripheral neuropathy has been a significant complication from these agents and they have not developed into clinically useful agents (Hall, E. J. Radiobiology for the Radiobiologist, 2nd edition, Harper and Row, Hagerstown, New York 1978; Coleman, C. N., Seminars in Oncology, 16:3 169-75, 1989).
Radioprotection research has been directed mainly at free radical scavenging compounds based on the sulfhydryl (--SH) group (Hall, S. J. Radiobiology for the Radiobiologist, 2nd edition, Harper and Row, Hagerstown, New York 1978). Although early studies suggested these agents would provide relative protection of normal tissue compared to tumours, they also have not been shown to be clinically useful (Hall, E. J. Radiobiology for the Radiobiologist, 2nd edition, Harper and Row, Hagerstown, New York 1978; Coleman, C. N., Seminars in Oncology, 16:3 169-75, 1989). A major problem has been the necessity to preload the patient with these compounds to obtain adequate tissue levels given that most of these agents are metabolized and rendered inert by the liver (Hall, E. J. Radiobiology for the Radiobiologist, 2nd edition, Harper and Row, Hagerstown, New York 1978).
Superoxide dismutase (SOD) and catalase (CAT) are two intracellular enzymes which function to convert superoxide (O.sub.2.sup.-) to peroxide (HO:OH--essentially two hydroxyl radicals) and peroxide to water (Southorn, P. A., Mayo Clin. Proc. 63:381-389 (1988); Petkau, A., Br. J. Cancer 55:Suppl VIII, 87-95 (1987); Fridovich, I., Autor, A. P. ed pathology of Oxygen Academic Press, New York, 1982, pp. 1-20; McCord, J. M., J Free Radic Biol Med 1986:2, 307-310; McCord J. M., Science 1974:185, 529-531) previous studies have shown that SOD exerts a protective effect against free radical mediated injury from a variety of sources, including radiation (Clark I. A., Pathology, 1986:18,181-186; McLennan, G., Autor, A. P. ed Pathology of Oxygen, Academic Press, New York, 1982, pp. 85-97; McCord, J. M., J Free Radic Biol Med, 1986; 2, 307-310; McCord, J. M. et al, Autor, A. P. ed Pathology of Oxygen Academic Press, New York, 1982, pp.75-83Petrone, W. F. et al, Proc Natl Acad Sci USA, 1980:77, 1159-1163; Autor, A. P. Life Science, 1974:14, 1309-1319; McCord, J. M., Fed Proc, 1987:46,2402-2406; Koyama, J. et al, Transplantation, 1985:40, 590-595; Atalla, SI. et al, Transplantation, 1985:40, 584-590; Manson, P. N. et al, Ann Surg 1983:198, 87-90;Petkau, A. et al, Biochem Biophys Res Commun, 1975:67, 1167-1174; Edsmyr, F., Autor, A. P. ed Pathology of Oxygen, Academic Press, New York, 1982, pp.315-326) although this has not been consistently found by all researchers (Westman, N. G. and Marklund, S. L., Acta Oncologica 1987:26, 483-487; Scott, M. D. et al, J. Biol. Chem. 1989:264(5), 2498-2501). CAT has been shown to provide a radioprotective effect in cell suspensions (Misra, H. P. and Fridovich, I., Arch Bioch Biophys, 1976:176, 577-581; McLennan, G. et al, Radial Res 1980:84, 122-132), and--more recently--in an in-vivo model (Jones J. B. et al, J Otolaryngol, 1990:19, 299-306).
Jones et al (Jones J. B. et al, J Otolaryngol, 1990:19, 299-306) studied the effect of SOD and CAT on radiation injury to rat skin. CAT was found to markedly ameliorate the acute radiation changes to rat epidermis and dermal vascular endothelium. SOD was found to have no protective effect by itself. The combination of SOD and CAT provided radioprotection similar to CAT alone--the addition of SOD had no additional beneficial effect.